1. Field of the Invention
The present invention relates to an optical device to correct the image of the pupil of the eye given by a spherical concave mirror. More specifically, a device according to the invention can be used to observe an image that is corrected of distortion due to a spherical or substantially spherical concave mirror that is tilted with respect to the direction at which the eye observes this mirror.
2. Discussion of the Background
The invention can be applied to a helmet visor especially but not exclusively for the pilot of an armed aircraft or helicopter or for the operator of a training simulator.
A helmet visor is an image-presenting device that is integrated into a helmet. The visor enables the wearer of the helmet, for example the pilot of an aircraft in flight, to observe visual information simultaneously with the view of the landscape or of the pilot""s cabin, which he perceives most usually through a protective visor.
The presentation of appropriate information, for example in the form of symbols, provides assistance in piloting and navigation. Thus, for armed vehicles, the presentation of a reticule provides assistance in the aiming of a weapon.
The information may also consist of an image of the landscape acquired by sensors other than the eye of the helmet wearer such as infrared sensors or visible light intensifiers to complement or replace direct viewing.
Inside the helmet, an image generator comprises an imager, for example a cathode-ray tube screen or a liquid crystal screen on which an image is formed.
The helmet most usually has a relaying optic to convey this image up a combiner which presents the conveyed image in a state where it is superimposed on the view of the landscape.
In order that the pilot may simultaneously observe the landscape which is viewed directly at infinity and the image from the imager, the latter is also focused at infinity by a collimation optic.
When the combiner is formed by a simple semi-reflective flat plate, the collimation of the image can be achieved by an optic placed between the combiner and the imager. A prior art embodiment of this kind has the major drawback of requiring a collimation optic that takes up far too much space in relation to the restricted field of view that is obtained.
To reduce the space requirement, a combiner with optical power has been proposed. A combiner of this kind provides its user with both the collimation of the image and the superimposition of the collimated image on the view of the landscape.
The prior art has a very rich variety of devices comprising a combiner with optical power. First of all, a concave spherical mirror achieves an average quality collimation of an image placed at a particular point in space located on the axis of the mirror and at a distance from this mirror equal to half of its radius of curvature. By placing an imager at this point, the eye located on the axis of the mirror receives rays coming from the imager after they are reflected on the spherical mirror, these rays being parallel and leading to the perception, by the eye, of a collimated image. If, furthermore, the mirror is semi-reflective, it enables the same eye to observe the landscape by transparency. However, in a device of this kind, the imager is on the axis of the semi-transparent spherical mirror and it conceals the field of view of the user.
To clear the user""s view, the spherical mirror may be tilted with respect to the normal to his/her face, and the user""s eye is no longer on the axis of the mirror. This tilting has the major drawback of leading to a collimated image that is affected by optical aberrations, especially off-centring aberrations, excessively limiting the use of such a device.
In order not to conceal the field of view of the user while at the same time limiting aberrations, the prior art teaches us the use of a parabolic mirror instead of a spherical mirror. The imager is placed at the focal point of the paraboloid described by the mirror and the eye observes the mirror along a parallel to the axis of revolution of the paraboloid.
The collimated image perceived by the eye has no spherical aberration but remains affected mainly by a coma with highly penalizing effects, the extent of which increases very rapidly with the field. Thus, the imager, while being off the axis of the field of view, remains a hindrance in the field of view.
One improvement consists of the exploitation of a double reflection on the parabolic mirror with an intermediate plane mirror placed at the level of the user""s forehead and called an onward reflection mirror. The two reflections are located on either side of the axis of the paraboloid. They make it possible to obtain a collimated image that is free of coma and whose other aberrations remain acceptable for a field of view that is still fairly restricted.
The desire to reduce the hindrance due to the onward reflection mirror has led to a development of the prior art. A device using a parabolic mirror and double reflection exhibiting asymmetry with respect to the axis of this mirror has been described. While this device reduces the hindrance in the field of view, it increases the aberrations, especially astigmatism. The device described comprises lenses that are tilted to reduce astigmatism. It also comprises a field lens to compensate for the field curvature and compensates for the distortion by a deformation of the image during its generation: the image is formed on the screen of the cathode-ray tube of the imager with a distortion that is the reverse of that which it is forced to undergo when crossing the optical device.
Furthermore, the initial idea of collimation by a spherical mirror has undergone new developments. Thus, a device has been described with a semi-transparent spherical mirror having a tilted axis, comprising a prism to compensate for the inevitable aberrations induced.
The prism is placed on the path of the light rays between the imager and the spherical mirror. The aberrations are minimized overall by adapting the tilt and aperture of the prism. And the astigmatism is corrected by an additional optical element that must be cylindrical.
This device is essentially penalized by a small field.
In parallel with this, devices have been made with spherical mirrors having no tilt in relation to the axis of view and with a shift of the imager.
A device of this kind has a semi-reflecting plane mirror placed between the spherical mirror and the user""s eye, at the focal point of collimation of the spherical mirror.
From the imager to the user""s eye, a light ray follows an optical path where, successively, it strikes the semi-reflecting plane mirror a first time, is reflected from this plane mirror towards the spherical mirror, and is then reflected on this spherical mirror and sent back to the plane mirror, it strikes the plane mirror a second time and goes through it to meet the eye.
The collection of spherical and plane mirrors is transparent for rays emitted by the landscape.
This type of device presents a high quality collimated image.
However, this design which implies a compromise between reflection and transmission by the plane mirror, has the drawback of sending to the eye only a small part of the initial light intensity and of thus too severely limiting the conditions of use of a helmet visor fitted out with this device.
The transmission of the useful image to the eye can be improved by slightly tilting the spherical visor with respect to the axis of view of the user and by subjecting a plane mirror to an anti-reflective treatment that is selective as a function of the angle of incidence of the light rays.
With this geometry, the first and second angles of incidence of one and the same light ray on the plane mirror have distinct angular values whereby the selective anti-reflective treatment, by circumventing the standard compromise between the reflection and the transmission of a ray that penalizes the above device, helps the reflection of the initial ray jointly with the transmission of the already reflected ray.
This device has a fairly wide field of view but is affected by aberrations due to the tilting of the axis of the spherical mirror. Certain aberrations are corrected by a tilted field lens and by spherical lenses.
The astigmatism and distortion are not excessive since the tilt is small, but are not corrected optically. Only a compensation of the distortion by a deformation of the image generated on the cathode-ray tube screen can be considered.
This device has improved luminosity, however the presence of the plane mirror between the eye and the spherical mirror most usually integrated into the visor of the helmet is a major drawback were it nor for the comfort and security of the eye on the one hand and for the high cost of its anti-reflective treatment on the other hand.
The problem is to make a device for presenting images for helmets with a spherical visor where there is no element interposed between the eye and the visor and which presents a collimated image that is satisfactory for the user, namely an image that is devoid of troublesome aberrations and has a wide field of view greater than or equal to 40xc2x0.
The use of a spherical part of the visor as a collimation element leads to major aberrations which must be corrected at least partially.
This is why the invention proposes an optical device for a system for presenting collimated images to a user comprising an imager and a substantially spherical off-axis concave mirror characterized in that the optical device comprises an optical axis and an aspherical concave mirror tilted on the optical axis, the intersection of the aspherical concave mirror with the plane of incidence of the optical axis being a curve with a variable radius of curvature to correct the distortion of the image presented to the user, said distortion being due to the substantially spherical off-axis concave mirror.
The light rays coming from the centre of the imager form the central field of the imager. The optical axis of the device corresponds to the path of the ray from the central field which passes through the centre of the user""s pupil.
The optical axis is most usually a broken line. For example, if the image is presented to the user straight in front of him, the part of the optical axis located between the eye and the spherical mirror is supported by a first straight line normal to the centre of the user""s pupil, the optical axis has a break at the intersection of this first straight line with the spherical mirror, and the image that the spherical mirror gives of this first straight line supports the next segment of the optical axis.
The aspherical concave mirror placed between the spherical mirror and the imager is tilted with respect to the optical axis. The surface of the aspherical concave mirror is preferably a second-order or quadratic surface.
If the invention is presented without a folding mirror, it is always possible, after the theoretical positioning of the various optical elements of the invention, to add one or more plane mirrors which introduce no aberration but make it possible to meet the constraints of space requirement, for example so that the device is adapted to the profile of the user""s head. An optic presented without folding mirror is called an unfolded optic.
The plane of symmetry of the unfolded optic contains the normal to the entrance pupil of the user""s eye and the centre of the spherical mirror.
The intersection of this plane with the aspherical concave mirror is a plane curve with a radius of curvature that is variable such as a non-degenerate conic. The surface of the aspherical concave mirror is used to correct the distortion of the image presented to the user that is due to the off-axis spherical mirror.
In one embodiment of the invention, the optical surface of the tilted aspherical concave mirror is supported by a paraboloid. In another embodiment, the aspherical concave surface rests on an ellipsoid.
The surface of the aspherical concave mirror is preferably a part of a surface of revolution. In this case, it has the advantage of being easier to make.
The axis of revolution of the aspherical concave mirror is located in the plane of symmetry of the unfolded optic.
The off-axis image of the pupil of the eye is the first pupil image of the device, it is tilted with respect to the optical axis. From this first tilted pupil image, the aspherical mirror according to the invention gives a second pupil image rectified on the optical axis.
In the case of a paraboloid of revolution, the axis of revolution of the aspherical concave mirror is substantially parallel to the normal to the first pupil image.
The device also comprises a power lens substantially centred and placed between the spherical mirror and the aspherical concave mirror.
When the surface of the aspherical concave mirror is described by a paraboloid, the power lens conjugates the image of the pupil of the user""s eye, given by the spherical mirror, in the vicinity of the focal point of conjugation of the paraboloid.
The ellipsoid has the advantage of having two focal points at finite distance. A surface of this kind is less easy to machine than a paraboloid but it achieves better correction because the ellipsoidal mirror is completely stigmatic for its two focal points and provides efficient conjugation of the vicinities of the focal points.
The invention has the advantage of correcting the distortion of the image presented to the user""s eye for a wide instrument pupil, for example with a diameter of at least 15 millimetres, and for a wide field typically greater than 40xc2x0. The instrument pupil is the zone of space in which the user of the instrument has to place the pupil of his eye in order to use the instrument.
This correction is particularly beneficial when a distortion cannot easily be imposed at the imager. Indeed, the prior art teaches us that in order to correct the distortion of the image given by an optical assembly, it is necessary to introduce a reverse distortion at the imager by electronic correction. This is easily done when the imager has a cathode-ray tube but this approach is not suited to an imager such as for example a light intensifier which does not have the necessary adjustments of the image.
The invention can be integrated into a helmet visor having a wide instrument pupil and a wide field.
Other features and advantages of the invention shall appear from reading the following detailed description of a particular embodiment made with reference to the following appended drawings in which the optical diagrams are shown unfolded, namely without a plane mirror.